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5e3 build hum/hiss troubleshooting

Discussion in 'Shock Brother's DIY Amps' started by choosebronze, Jul 28, 2020.

  1. tubegeek

    tubegeek Friend of Leo's

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    No, the noise sources are the grid leak and plate load resistors. A 100R or 1K or 5K CC in series with the signal is not a significant contribution.
     
  2. tubegeek

    tubegeek Friend of Leo's

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    Apologies, @Tom Kamphuys - I neglected the noise contribution of the big 68K input resistor in the standard Fender input-jack arrangement, which does indeed add its major share of noise as well. My bad!
     
  3. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Thanks guys, lots to digest in these recent posts. I'll review them all and try your suggestions and see what happens. If I connect the grid resistors directly to the tube socket, I should run shielded wire from the jack to the resistor, grounded at one end, yea? The only thing that makes me skeptical of those resistors as a source of the problem is that the 68K's were originally carbon resistors, replaced with metal film, and the issue is identical. If those resistors were adding the noise, it would be odd to have no worsening/improvement, wouldn't it? But hey, I'll give it a shot, can't hurt.

    As for what kind of resistors the new ones are, the NTE package only says "1/2w flameproof resistor." I asked at the store to confirm they were metal film, and the guy said they were, but maybe he was wrong. Resistors are cheap, I can order different ones online.
     
  4. D'tar

    D'tar Friend of Leo's

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    For some reason I can not listen to your file....

    Is the noise still present if you remove V1? If yes you can move beyond the input. If no... ground the v1a grid right at the socket. Noise gone?
     
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  5. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    I was able to move the 68k resistors directly to the socket using shielded wire (grounded at the jacks). It's hard to be exact but I'd say it didn't change anything. I elevated the heater CT to the location suggested. That did seem to help that underlying 60 cycle hum A LOT (awesome perk along the way!) but didn't help the static hiss.

    Chop sticked the jacks pressing the contacts together, that didn't change anything. Only thing I haven't confirmed is whether these resistors are metal film or metal oxide. I'll pick up some metal films for sure and try those directly on the sockets. I think the only other suggestion I haven't tried yet is metal film at the plate resistors. I can do those too.

    No, the static noise stops if I remove V1. Not sure what you mean about grounding V1A at the socket though.


    A thought that just popped in my head - I'd expect this chassis to be a bit noisier out in the open like it is. Once it's in a cabinet there will be shielding on one side. Is it crazy to stick some shielding on a piece of plywood and rest it on top of the open chassis to see if that changes things?

    Thanks for all the help so far. This is the most stubborn issue I've had in a long time and you're keeping me from going crazy.
     
  6. D'tar

    D'tar Friend of Leo's

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    Not crazy! Cookie sheet, etc, is common.

    Using a jumper wire. One end connected to ground. Touch the other end to v1 control grid, pin 2 right at the socket.
     
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  7. tubegeek

    tubegeek Friend of Leo's

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    not crazy +1
     
  8. Paul G.

    Paul G. Friend of Leo's

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    My 2 cents.

    Hum comes from one of three sources: Bad filter caps -- unlikely as yours are new, but as a last resort. Bad ground connection, missing ground connection -- most likely source. Ground loop -- if you followed a known layout less likely but worth checking. Filament wiring -- unbalanced, not center tapped (from PT or artificial), proximity to signal wiring. Finally AC and DC running too close together somewhere.

    Microphonics: bad solder joint.


    Hiss: bad solder joint, bad tube, plate resistors (mainly in an old amp, but a resistor that has been overheated by soldering may get noisy). Freeze spray can eliminate the resistors as a source.
     
  9. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Cool thanks guys, I'll do more troubleshooting tomorrow night.

    Paul G, sounds like the common thread between all of those is a bad solder joint, so I'll probably have to go hunting for one...
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    As long as you are fussin' with all the rest of this circuit, check the V1 socket tension and lightly scrape the tube pins with an exacto blade to assure good electrical contact... again.

    *lightly scrape* means just take the oxidation off. There should be no metal filings left on the ground after this procedure.:)

    You may want to try this with other tubes in V1 as well... again.
     
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  11. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    That drastically takes away the static hiss on the Normal channel. I wouldn't expect it should have any effect on the Bright channel. With pin 2 jumpered to ground I only hear the static faintly with the input fully at 12. I'd be more shocked if I didn't hear any static with the volume maxed. So what does that tell me? That the static is an issue with the inputs being grounded? Isn't it weird that the hiss would be happening on both Bright and Normal channels? That means I have grounding problems with both pairs of input jacks?

    Worth noting, grounding pin 2 has no effect on the microphonic V1. I was thinking about Lowerleftcoast's comment (hey, I live there too). I've tightened 8 pin sockets before, but I can't see any way to check the 9 pin sockets. The ceramic base just has tiny holes for each pin, there's no place to nudge the socket itself. But in inspecting it, I noticed the tube is REALLY wobbly in the socket. Not hard to just try another socket I guess, though I managed to run out of solder tonight so that'll have to wait.
     
  12. D'tar

    D'tar Friend of Leo's

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    @choosebronze, just trying to narrow down the source of your issue. Now you can ground v1 pin 7 (v1b control grid). The channels are interactive so playing with each volume can help isolate as you are finding. I'm not against anything the fellows above have recommended, I simply have never seen a 5e3 circuit with a standard layout have an issue requiring additional components.
     
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  13. D'tar

    D'tar Friend of Leo's

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    Did you do this? I didn't see a response.

    Can we get a clear straight on photo of the whole board.

    Also voltages would be nice.
     
  14. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    And a nice cold beer? :)
     
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  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    FWIW, the NTE site claims they are metal film.
     
  16. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Yes, sorry, the PT CT and filter cap ground go to the same PT bolt. I'll take some full board pics and measure voltages and report back.
     
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  17. tubegeek

    tubegeek Friend of Leo's

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    I forget if this has been covered yet, but: nothing else should connect on the ground side of that loop. In practice that means that the wire from the cap (-) should not have any other connections "y" -ing onto it from the other grounds. Most builders make a solid chassis ground connection near the input jack and run a ground bus to all other grounds from there. A possible exception would be the power amp's ground connections which might be convenient to run to the power transformer bolt also - but NOT to the wire from bolt to cap, all the way to the bolt.
     
  18. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Oh man. Standing over that chassis gets WARM. This amp will be easier to work on in winter...

    Here are some pictures and voltages. Things to note: 1. I left the old input wires under the board for now, they aren't connected to anything, if you're wondering why they're just floating there. 2. All the purple/yellow over by the input jacks are RG174/shield grounds for resistors on the tube sockets. 3. Yes, if I end up keeping the grid resistors on the sockets I will clean up the resistor wiring, it looks pretty ugly right now but that was just done a couple days ago, all symptoms are the same with the "standard" layout I started with. 4. The filter caps are all the same specs - just had 2 different brands.

    Here are the voltages, they all look good to me. All heater pins measure 3.12VAC (marked H on chart).


    Screen Shot 2020-08-03 at 11.06.35 PM.png
    IMG_4131.JPG IMG_5842.JPG IMG_7854.JPG
     

    Attached Files:

  19. Paul-T

    Paul-T Tele-Holic

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    Others with mroe experience might disagree with me but there are a lot of wire loops there. Purple on to 12AX7, red/white, purple around the controls.

    When I was an eletronics student I made a vision-processing circuit on a breadboard for my thesis and had one looped wire around one section. It attracted EM radiaiton, oscillated, and acted as a kind of high-pass filter for the whole circuit (ironically, it performed a useful function).

    f you flatten or reroute those wires, run them like King Fan's or D'tar's layouts, running where possible alongside the chassis which will short out any EM radiation, it will cut out any possibility of that, which could definitely introduce white noise. There are about six main offenders, it won't take long, can't harm and might well help. You could chopstick each one, but there might even be chaotic interplay between them.
     
    Last edited: Aug 4, 2020
  20. D'tar

    D'tar Friend of Leo's

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    Thanks for the voltages and pictures....

    Is this soldering damage here.... From personal experience.... This is unfortunate but may contribute to your issue.

    upload_2020-8-4_9-7-26.png
     
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